1. Field of the Invention
The invention relates to resonant circuits and in particular to voltage-controlled oscillators.
2. Background of the Invention
Voltage-Controlled Oscillators (VCOs) are used in many systems, such as communications systems and computers, where frequencies are synthesized. Frequency synthesis can be used, e.g., to provide a carrier frequency for a signal in a communications system such as a radio transceiver.
VCOs are configured to have an oscillation, or resonant, frequency. For oscillation frequencies higher than about 1 GHz, typically LC oscillators are used because they have low noise and are relatively stable. LC oscillators typically use a tank circuit including an inductance (L) and a capacitance (C) connected in series or in parallel to provide a resonant circuit. The oscillation frequency of the LC tank depends on a product of the inductance and capacitance (the LC product) of the tank.
A control voltage called the tuning voltage is used in a VCO to adjust the oscillation frequency. The oscillation frequency can also be adjusted by varying the capacitance of the tank using the tuning voltage. This may be accomplished by implementing the tank capacitor as a varactor, whose capacitance varies with the tuning voltage. The tuning voltage has a range of voltages that can be provided, corresponding to a range of capacitances that can be provided. This range of capacitances corresponds to a range of frequencies producible by the LC tank. The varactor is typically designed to have a desired nominal capacitance, so that the LC tank will oscillate at a desired frequency, when the tuning voltage is at a nominal voltage. The nominal voltage will be approximately in the middle of the tuning voltage range if the change in oscillation frequency is linear relative to the change in tuning voltage.
A number of technical advances are achieved in the art, by implementation of an LC VCO for compensating variances in capacitance of an LC tank of the VCO. The LC VCO may be broadly conceptualized as a system in which the inductive portion of an LC tank of the VCO is selectively adjusted, thus helping to ensure that an LC product of the LC tank is within a desired range to help ensure that the VCO can output a desired frequency or frequencies.
For example, an LC VCO may utilize a system architecture in which inductance is adjusted to compensate for variances between actual capacitance and designed/desired capacitance to achieve a desirable LC product. An implementation of the system architecture may include selectable inductances for connection to a capacitor of an LC tank of the VCO. Inductances of different values can be connected to the capacitor to effectively alter the capacitance, by actually altering the LC product. The different inductance values can be provided by different lengths of bondwire, or by different-valued, coils, inductors and the like, or by combinations of these techniques.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the appended claims.